NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Battery Thermal Characterization

P.I.: Matthew KeyserLab Lead: Ahmad PesaranOther Contributors: Aron Saxon, Mitchell

Powell, and Ying ShiNational Renewable Energy LaboratoryJune 7, 2016 Project ID # ES204

This presentation does not contain any proprietary, confidential, or otherwise restricted information.

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Overview

• Project Start Date: 10/2004• Project End Date: 9/2018• Percent Complete: Ongoing

(Supporting ongoing DOE/USABC battery developments)

• Decreased battery life at high temperatures.• High cost due to an oversized thermal

management system.• Cost, size, complexity, and energy

consumption of thermal management system.• Decreased performance at low temperatures.• Insufficient cycle life stability to achieve the

3,000 to 5,000 “charge-depleting” deep discharge cycles.

• Total Project Funding:o DOE Share: 100%o Contractor Share: 0%

• Funding Received in FY15: $500K• Funding for FY16: $500K

Timeline

Budget

Barriers

• USABC – GM, Ford, Fiat-Chrysler (FCA)• JCI• Leyden• LGCPI• Maxwell• Saft• SK Innovation• Seeo

Partners

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Relevance of Battery Thermal Testing and Modeling

Objectives of NREL’s work

• To thermally characterize cell and battery hardware and provide technical assistance and modeling support to DOE/U.S. DRIVE, USABC, and battery developers for improved designs.

• To enhance and validate physics-based models to support the thermal design of long-life, low-cost energy storage systems.

• To quantify the impacts of temperature and duty cycle on energy storage system life and cost.

USABC = U.S. Advanced Battery ConsortiumU.S. DRIVE - United States Driving Research and Innovation for Vehicle Efficiency and Energy

Life, cost, performance, and safety of energy storage systems are strongly impacted by temperature

as supported by testimonials from leading automotive battery engineers, scientists, and executives.

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Milestones

Month/ Year

Milestone or Go/No-Go Decision

Description Status

9/2015 Milestone Report on thermal evaluation of advanced cells and battery packs

Complete

12/2015 Milestone Present thermal data at USABC technical review meetings

Complete

3/2016 Milestone Report on battery thermal data for USABC cells

Complete

6/2016 Milestone Present thermal data at USABC technical review meetings

On Track

9/2016 Milestone Report on battery thermal data of USABC battery cells/packs

On Track

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Thermal Testing – Approach

• NREL provides critical thermal data to the battery manufacturers and OEMs that can be used to improve the design of the cell, module, and pack and their respective thermal management systems.

• The provided data include infrared imaging results and heat generation of cells under typical profiles for HEV, PHEV, and EV applications.

EV = electric vehicle; HEV = hybrid electric vehicle; OEM = original equipment manufacturer; PHEV = plug-in hybrid electric vehicle; HWFET = Highway Fuel Economy Test;UDDS = Urban Dynamometer Driving Schedule

Tools• Calorimeters• Thermal imaging• Electrical cyclers• Environmental

chambers• Dynamometer• Vehicle simulation• Thermal analysis

tools

Measurements• Heat capacity• Heat generation• Efficiency• Thermal performanceo Spatial temperature

distributiono Cell-to-cell temperature

imbalanceo Cooling system

effectiveness

Test Profiles• Normal operation• Aggressive operation• Driving cycles

o US06o UDDSo HWFET

• Discharge/charge rateso Constant current (CC)o Geometric charge/dischargeo FreedomCAR profiles

Cells, Modules, and Packs

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Thermal Testing – ApproachThermal Imaging• Temperature variation across cell• Profiles: US06 cycles, CC discharge/charge• Unique testing method reducing environmental

impacts

• Results reported to DOE, USABC, and battery developers Photos by Kandler Smith, NREL

Thermal Management Performance• Temperature variation across pack under

realistic conditions• Assessing vapor compression, air, and liquid cooling

systems• Profiles: US06 cycles, CC discharge/charge

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Heat Generation and Efficiency – ApproachTo

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Hea

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RMS Current (A)• Heat generation, heat capacity, and efficiency• Test temperature range: -30°C to +45°C• Profiles: USABC and US06 cycles, CC

Photo by Dennis Schroeder, NREL

Using state-of-the-art isothermal battery calorimeters

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Efficiency Comparison of Cells Tested in FY15 and FY16 at 30°CUnder Full Discharge from 100% to 0% SOC

Technical Lessons Learned

Typical Chemistries

Experimental Chemistries: Lithium Anode, Mn Rich, Silicon Anode

SOC – state of charge

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Heat Rate Comparison of Cells Tested in FY15 and FY16 at 30°CUnder Full Discharge from 100% to 0% SOC

Technical Lessons Learned

SOC – state of charge

Heat Rate – Used to size thermal management system

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Lithium Cell Efficiency at 30°C, 0°C, and -15°CUnder Full Discharge from 100% to 0% SOC

Technical Lessons Learned

Efficiency is highly dependent on temperature.Thus, heating and cooling systems may be necessary.

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EV Cell Gen1/Gen 2 Efficiency ComparisonTechnical Lessons Learned

Standard Chemistry

Gen 1

Gen 2

During the USABC programs, electrochemical and mechanical design changesare made to the cells. Calorimetry can determine if the changes are positive.

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Low-Current Entropic Heating

Heat in a cell is produced by: • The resistance of the various cell components (electrode, cathode, anode, etc.);

this is known as Joule heating, which can be minimized by cycling the cells at low currents

• Entropic reactions within the cell—exothermic and endothermic reactions within the cell due to the transfer of ions and electrons.

Cycling the batteryat the inflection points may cause cracks in the anode or cathode, which may lead to decreased performance and life.

Technical Lessons Learned

In general, Joule heating is an order of magnitude less than the entropic heating.

Zero Axis

Exothermic

Endothermic

Inflection Point

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Titanate Anodes Limit Phase TransitionTechnical Lessons Learned

Titanate anodes do not experience the samephase transitions as graphite anodes.

NMC = nickel manganese cobalt

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Calorimetry Testing Can Identify Entropic Heating/CoolingTechnical Lessons Learned

Full charge/discharge: 100% SOC 0% SOC

Entropic heating has an effect at lowcurrents

Entropic heating has less of an effect at high currents

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Heat Generation under Various Drive Cycles

Cycle RMSCurrent(amps)

Efficiency (%)

Heat Rate (watts/cell)

DST 4.9 95.3 0.5

70% Power USO6 8.0 93.9 1.2

100% Power US06 11.8 91.1 2.6

Calorimeter can measure the efficiency and heat generation under various drive cycles—helps in designing thermal management systems for battery packs.

Cycle type can have a large effect on efficiency and heat rate.

Technical Lessons Learned

Calorimeter Trace

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PHEV/EV Cell at End of 2C Constant Current Discharge

L04

L01

L03

L02

Large temperature variations across surface of cell– affects life and performance

Technical Lessons Learned

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Pack Thermal Temperature Studies

Measured temperature rise, temperature uniformity, and parasitic losses versus temperature and duty cycle, extrapolating calendar life for different scenarios with and without active cooling.

A123

JCS

LGCPI

Technical Lessons Learned

Photos by Dirk Long, NREL

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Thermal Management System Performance Under a PHEV CD/CS Drive Cycle

Technical Lessons Learned

End of CD

Cooling Plate

Approximately 100 cells in pack

CD = charge depletingCS = charge sustaining

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Thermal Management System Performance Under a PHEV CD/CS Drive Cycle

Technical Lessons Learned

End of CD Cycle End of CS Cycle

Test AmbientTemp.

(oC)

MaximumTemp.

(oC)

DeltaTemp.

(oC)

MaximumTemp.

(oC)

DeltaTemp.

(oC)

US06 PHEV CDCS 30 45 12.1 29 8.4

US06 PHEV CDCS 40 52 12.2 30 9.7

• The recent U.S. DRIVE RFPI limits the cell-to-cell temperature in a PHEV pack to less than 3oC. In this pack the cell-to-cell temperature difference is greater than 12oC.

• If not properly designed, thermal management systems can cause a large cell-to-cell temperature spread. These temperature differences affect the cycle life of each cell, potentially resulting in warranty issues.

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Responses to Previous Year Reviewers’ Comments

Reviewer Comment: The reviewer found the entropic studies quite interesting to learn about, but was unsure how that will lead to improvements in design. The reviewer also noted that publications in peer-reviewed literature were not included in the list of accomplishments, asking if this was not important for the project.

Presenter Response: The entropic studies give an indication of where the battery can be cycled to avoid mechanically fracturing the cathode and anode. When a cell is cycled from 100% to 0% SOC, the cathode and anode material can go through several morphology changes. The changes will coincide with the contraction/ expansion of the active material. To limit the mechanical stress on the active material, the battery manufacturer or end user can choose to limit the amount of time at these phase transition points, thereby extending the life of the battery. NREL’s calorimeters will indicate where these phase transitions occur.

As for peer-reviewed publications, we are required to keep the information generated under this task confidential/proprietary for a period of 5 years. This requirement limits our ability to publish in peer-reviewed journals.

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Responses to Previous Year Reviewers’ Comments

Reviewer Comment: While several partners were listed, the reviewer noted, it was not clear how the PIs are working with those partners. For example, the reviewer said, it was unclear if the PIs are testing batteries from all or some of the partners.

Presenter Response: We work with USABC/U.S. DRIVE (GM, Ford, FCA) to identify promising battery technologies and then engage the manufacturers to develop these technologies for HEV, PHEV, and battery EV applications. Other than U.S. DRIVE, we have worked and tested the battery technologies for each of the other partners listed in the presentation and poster. We typically present our results each quarter to the battery manufacturers and show how their technology can be effectively utilized and/or improved to meet the goals for the specific program.

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Collaborators• USABC partners Fiat-Chrysler (FCA),

GM, and Ford• USABC Contractors – Technology

Evaluated at NRELo JCIo Leydeno LGCPIo Maxwello Safto SK Innovationo Seeo

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Remaining Challenges and Barriers

• Address life issues at high and low temperatures—15-year target.

• Address high energy storage cost due to battery packaging and integration costs.

• Reduce the cost, size, complexity, and energy consumption of thermal management systems.

• Optimize the design of passive/active thermal management systems—explore new cooling strategies to extend the life of the battery pack.

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Proposed Future Work• Continue thermal characterization for DOE, USABC, and partners

o Cell, module, and subpack calorimeters are available for industry validation of their energy storage systems.

• Develop battery usage models with the calorimeter heat generation data that will predict the thermal performance of energy storage systems under various drive cycles and environmental conditions—models to be utilized by GM, Ford, Fiat-Chrysler (FCA), and battery developer(s).

• The data will be used to enhance physics-based battery models in conjunction with DOE’s Computer-Aided Engineering for Automotive Batteries (CAEBAT) program.

• Continue to develop and evaluate liquid, air, and vapor compression thermal management systems to extend the energy storage cycle life.

• Work with OEMs and battery manufacturers to identify:

o The best solutions to reduce the cell-to-cell temperature variations within a pack in order to extend life.

o Minimize parasitic power draws due to the thermal management system.

o Investigate new solutions for the thermal management of batteries – phase change material, new refrigerants, etc.

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Summary

• We collaborated with U.S. DRIVE and USABC battery developers to obtain thermal properties of their batteries.o We obtained heat capacity and heat generation of cells

under various power profiles.o We obtained thermal images of the cells under various

drive cycles.o We used the measured results to validate our thermal

models.o All the data have been shared with the battery developers.

• Thermal properties are used for the thermal analysis and design of improved battery thermal management systems to support and achieve life and performance targets.

Technical Back-Up Slides

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Efficiency Comparison of Cells Tested at 30°C and 0°C under Full Discharge from 100% to 0% SOC

Testing the efficiency of cells at multiple temperatures shows how different additives/designs will affect performance.

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0 0.5 1 1.5 2 2.5

Efffi

cien

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)

C Rate

30oC

0oC

Cell A Cell B

Cell C

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Efficiency Can Change After Limited Battery Use

• Fuel economy standards are increasing.

• In the U.S., the fuel economy of a vehicle is determined by the EPA.

• The calorimeter can determine if the vehicle battery has a “break-in” period – in other words, the battery efficiency increases after cycling the battery.

• Knowing how your battery performs over time may prevent/reduce EPA fines for not meeting future fuel economy standards.

Limited Cycling

Beginning of Life

Not typical of all energy storage systems

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Efficiency Comparison of Successive Generations of Cells

Gen 1

Gen 2

Full Discharge – 100% to 0% SOC:Testing over the entire discharge range of

the cell gives the impression that the second-generation cell is less efficient.

Partial Discharge – 70% to 30% SOC:Testing over the usage range of the cells

shows that they have approximately equal efficiencies.

Important to test the cells over the SOC range in which they will be used.Gen 1

Gen 2

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Cell versus Module Heat Generation

Heat generated by interconnects is important to understand in order to properly design a thermal management system

Module heat generation/cell

Individual cell heat generation

RMS Discharge Current (Amps)

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New Chemistries – Titanate AnodesFor the same current, the cell is more efficientunder charge than discharge.

Charge impedance limiting Ah input at high rates